L = $20 r = 4% t = 2 years

I'm guessing this is a question about interest rates? If you have $20 that increases by 4% in one year, you need to multiply 20 by 1.04. This gets you $20.8.

If you are talking about compound interest, we will take this number and multiply it again by 1.04 for the second year. 20.8 x 1.04 = $21.632.

If it is instead simple interest, we will simply add another. 8 dollars for each year, instead of getting 4% interest compounded every year onto the new value. This gets you $21.6.